Open NetBIOS

Quick Info

Plugin TitleOpen NetBIOS
CategoryNetwork Security Groups
DescriptionDetermine if UDP port 137 or 138 for NetBIOS is open to the public
More InfoWhile some ports such as HTTP and HTTPS are required to be open to the public to function properly, more sensitive services such as NetBIOS should be restricted to known IP addresses.
Recommended ActionRestrict UDP ports 137 and 138 to known IP addresses

Detailed Remediation Steps

  1. Log into the Microsoft Azure Management Console.
  2. Select the “Search resources, services, and docs” option at the top and search for Network security groups.
  3. Select the “Network security group” that needs to be verified.
  4. Click on the “Name” of the selected “Network security group” to access the port configurations.
  5. In the “Overview” tab scroll down the page and check the “Inbound security rules” under “Settings”.
  6. If the “NetBIOS” for port 137 or 138 with protocol “UDP” is showing as “Allow” for all “Source” and “Destination” then the selected “Network security group” has UDP port 137 or 138 for NetBIOS is open to the public.
  7. Repeat steps number 2 - 6 to verify other “Network security group.”
  8. Navigate to the “Network security group” and select the security group that needs to modify for restricting the NetBIOS protocol on port 137 or 138 to specific IP Address.
  9. Scroll down the left navigation panel and choose “Inbound security rules” under “Settings."
  10. Click on the “NetBIOS” protocol in the “Name” column and under “Source” select the “IP Address” from the dropdown menu and enter the “Source IP addresses/CIDR ranges” as per the requirement, select the required Protocol and Action and click on the “Save” option at the top panel.
  11. Repeat steps number 8 - 10 to restrict UDP port 137 or 138 for NetBIOS to known IP addresses.